Long-Term Debt - Maturities Of Long-Term Debt (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 20, 2018	Jun. 30, 2017	Nov. 01, 2016
Debt Instrument [Line Items]
Less than one year	$ 47,888
One to two years	115,345
Two to three years	7,746
Three to four years	133,133
Four to five years	133,539
Over five years	28,258
Gross debt	465,909
Unamortized debt discounts	(27,289)
Unamortized debt issuance costs	(9,413)
Total debt	429,207
Less: current maturities	(47,888)	$ (47,252)
Long-term debt	381,319	393,063
Term Loan Maturing June 2020
Debt Instrument [Line Items]
Less than one year	5,147
One to two years	27,610
Two to three years	0
Three to four years	0
Four to five years	0
Over five years	0
Gross debt	32,757	33,693		$ 100,000
Unamortized debt discounts	0
Unamortized debt issuance costs	(310)
Total debt	32,447
Less: current maturities	(5,147)
Long-term debt	27,300
Convertible Senior Notes Maturing May 2022
Debt Instrument [Line Items]
Less than one year	0
One to two years	0
Two to three years	0
Three to four years	125,000
Four to five years	0
Over five years	0
Gross debt	125,000	125,000			$ 125,000
Unamortized debt discounts	(10,305)	(11,000)			$ (16,900)
Unamortized debt issuance costs	(1,633)
Total debt	113,062
Less: current maturities	0
Long-term debt	113,062
Convertible Senior Notes Maturing September 2023
Debt Instrument [Line Items]
Less than one year	0
One to two years	0
Two to three years	0
Three to four years	0
Four to five years	125,000
Over five years	0
Gross debt	125,000	125,000	$ 125,000
Unamortized debt discounts	(16,984)	(17,800)	$ (20,100)
Unamortized debt issuance costs	(2,738)
Total debt	105,278
Less: current maturities	0
Long-term debt	105,278
MARAD Debt Maturing February 2027
Debt Instrument [Line Items]
Less than one year	7,027
One to two years	7,378
Two to three years	7,746
Three to four years	8,133
Four to five years	8,539
Over five years	28,258
Gross debt	67,081	70,468
Unamortized debt discounts	0
Unamortized debt issuance costs	(3,903)
Total debt	63,178
Less: current maturities	(7,027)
Long-term debt	56,151
Nordea Q5000 Loan Maturing April 2020
Debt Instrument [Line Items]
Less than one year	35,714
One to two years	80,357
Two to three years	0
Three to four years	0
Four to five years	0
Over five years	0
Gross debt	116,071	$ 125,000
Unamortized debt discounts	0
Unamortized debt issuance costs	(829)
Total debt	115,242
Less: current maturities	(35,714)
Long-term debt	$ 79,528